[skip ci] Standardise transfer log payload with correlation_id

Implements Part 9a of #586550 (closed) — structured logging for namespace transfers. Adds a shared Namespaces::TransferLogging concern that builds a standardised payload (correlation_id, namespace_id, namespace_type, transfer_state, initiated_via, duration_s, queue_wait_s, retry_count, error_class, error_message) for transfer log entries in Groups::TransferService, Projects::TransferService, Namespaces::Groups::TransferWorker, Projects::TransferWorker, and Namespaces::TransferWorkerHelper. initiated_via, queue_wait_s, and retry_count are wired in as optional placeholders for now; threading initiated_via through controllers/REST/GraphQL is deferred to a follow-up to keep this change focused on the logging foundation. References: #586550 (closed) . /assign me /milestone %19.1

Related to #586550 (closed)